The present invention relates to a retractor for a vehicle seat belt.
A seat belt retractor typically has a spool onto which seat belt webbing is wound. The spool is mounted in a frame for rotation about the axis of rotation of the spool and is spring loaded in a direction to wind up the webbing to keep the webbing taut across a vehicle occupant. Modern seat belt retractors apply a gentle restraining force to the vehicle occupant but allow some movement for the comfort of the vehicle occupant. For example, allowing the vehicle occupant to lean forward to retrieve items from a glove compartment or to tune a radio.
At least one crash sensors is incorporated into the seat belt retractor to detect an emergency situation. When the crash sensor detects and emergency situation a mechanism is activated to lock the spool against rotation in a webbing protraction direction, thus securely restraining the vehicle occupant against forward motion.
The mechanism which effects this locking typically comprises a load bearing locking pawl pivotally mounted in the frame to engage a toothed ratchet wheel attached to the spool. The locking pawl is rotated into engagement with the ratchet wheel by interaction of a cam follower on the pawl with a cam slot in a control member. When an emergency is detected, the crash sensor causes the control member to be coupled to the spool so that the control member rotates. Thus, the cam moves and the cam follower slides along the cam surface causing the locking pawl to pivot into engagement with the ratchet wheel and lock the spool. The vehicle occupant is then securely restrained.
The cam is traditionally a smooth, gently curved surface along one side of an oval or curve shaped cut-out in the control member so that the locking pawl is brought into engagement in a smooth controlled manner. The geometry is arranged so that the engagement is correctly phased. For example, so that the pawl cleanly engages between teeth of the ratchet, because if it is brought into engagement tooth to tooth, at the high speed and high force typical of modern seat belt retractor, the teeth will not lock properly and are likely to shear.
A problem has recently been identified in modern seat belt retractors. Ever increasing speeds of operation are demanded by manufacturers to satisfy the ever more stringent safety requirements of the regulatory bodies. Modern seat belt retractors are used with pretensioners that actively retract a predetermined amount of webbing back onto the spool to take up slack in the webbing and more securely restrain the vehicle occupant before locking the retractor, or which cause the buckle of the restraint system to move down rapidly thus taking up any slack. Modern pretensioners use pyrotechnic firing mechanisms and are very fast. However, at high speeds of operation, the retractor components are subject to high dynamic loading due to the mass properties of the components and the forces necessary to accelerate them. This results in deformation of the components and causes a change in their kinematic response. As a result, the locking pawl tends to be engaged later as the speed of operation increases. In the extreme, the locking pawl will be out of phase with the ratchet wheel and will not lock properly or will cause components to shear, thus destroying the retractor. This is undesirable.
To overcome this problem, the phasing of the locking pawl engagement can be adjusted to be correct at high-speed operation. However, the seat belt retractor then fails to lock properly at low speed operation, which results in excess wear on the components at low speed and consequently early failure of the mechanism or jamming of the retractor.
The present invention provides an improved seat belt retractor that can operate effectively and reliably over a wide range of speed.
According to the present invention, there is provided a seat belt retractor comprising a spool rotatably mounted in a frame, a ratchet wheel rotatably mounted to rotate with the spool, a control member mounted in the frame, a locking pawl pivotally mounted in the frame to pivot into engagement with the ratchet wheel, a sensor responsive to an emergency, a mechanism for coupling the control member to the spool upon activation of the sensor, a cam surface and a cam follower coupling the locking pawl to the control member, wherein, the cam surface has a discontinuity arranged so that at low speeds of operation cam follower stays in contact with the cam surface and follows the cam surface to bring the locking pawl into engagement with the ratchet wheel, and at high speeds of operation the cam follower leaves the cam surface before the locking pawl engages with the ratchet wheel, in the region of the discontinuity because of the impulsive load generated between the cam follower and the cam surface to move the locking pawl into engagement with the ratchet wheel earlier than under low speeds of operation.